1. Field of the Invention
The present invention relates to optical communication equipment and, more specifically, to reducing effects of polarization mode dispersion (PMD).
2. Description of the Related Art
Polarization mode dispersion (PMD) occurs in an optical fiber as a result of small birefringence induced by deviations of the fiber's core from a perfectly cylindrical shape, asymmetric stresses or strains, and/or random external forces acting upon the fiber. Due to PMD, two polarization components of an optical signal corresponding to two principle states of polarization (PSP) travel in a fiber at different speeds and arrive at the receiver with a differential group delay (DGD). As a result, optical pulses corresponding to optical bits may be significantly distorted, resulting in errors at the receiver. In addition, PMD is a wavelength-dependent phenomenon. That is, the amount (level) of PMD imparted by the same optical fiber will generally be different for two optical signals simultaneously traversing that fiber when those signals correspond to different optical communication channels (wavelengths).
Several techniques have been proposed to date to mitigate the effects of PMD in optical communication systems. Typically, a device known as a PMD compensator is deployed at the receiver end of a fiber transmission link to improve the chances that the receiver correctly decodes PMD-distorted optical bits.
FIG. 1 illustrates an exemplary prior art PMD compensator (PMDC) 100. Compensator 100 comprises a polarization controller (PC) 102, a DGD element 104, a PMD monitoring unit (MU) 106, and control electronics 108. During operation, PC 102 receives a PMD-distorted optical signal and separates it into two PSP components. DGD element 104 subjects the faster PSP component to a compensating delay to realign it with the slower PSP component. The two PSP components are then recombined and directed to, e.g., a receiver (not shown) for decoding. The output of DGD element 104 is tapped and analyzed by MU 106, which is configured to provide feedback to PC 102 and DGD element 104 via signals 112 and 110, respectively, generated by control electronics 108. Based on those signals, PC 102 and DGD element 104 adaptively change their settings to correspond to the dynamically varying amount of PMD in the transmission link. Certain implementations of compensator 100 are described in commonly owned U.S. Pat. No. 5,930,414 by Fishman, et al., the teachings of which are incorporated herein by reference.
FIG. 2 illustrates a prior art multi-channel receiver 200 that is typically used in wavelength division multiplexing (WDM) communication systems affected by PMD. Multi-channel receiver 200 employs a plurality of PMD compensators 100, where each channel has a dedicated compensator. A WDM signal 202 applied to multi-channel receiver 200 and having n WDM components (labeled λ1-λn in FIG. 2), each corresponding to a different communication channel, is decomposed into n signals 206-1-206-n using a de-multiplexer (DMUX) 204. Each signal 206 is then processed by a corresponding PMD compensator 100, e.g., as described above, and decoded by a corresponding single-channel receiver 212 of a receiver array 210.
One problem with multi-channel receiver 200 is that it is relatively expensive to implement primarily due to the cost associated with multiple PMD compensators 100. This problem is progressively exacerbated due to the quickly increasing number of channels in modern WDM communication systems, often having 32 or more WDM channels and hence requiring a corresponding number of PMD compensators.